Copyright NEVER goes away, the scum bag suits of this world have made sure of that with the most ludicrous timescales for natural copyright expiration the slimey useless beings they are.
So to answer the question who owns the copyright to kickstart/workbench it will be the liquidator of a company that goes bankrupt and then either a separate purchaser or one of the creditors who may accept the copyright ownership in lieu of payment.
As for Cloanto....and anyone involved in that overpriced rubbish like Amiga Forever (and the even more lame and overpriced C64 Forever...which legally is on the same dubious ground as a DVD-R sold on ebay actually) they can go to hell to.
As far as copyright goes.....ALL companies should be forced to produce a physical product for sale in full packaging in a minimum quantity of 100,000 units or else relinquish copyright.
So if you host a game like say DonkeyKong.TAP on your website....and Nintendo issue you a warning to remove it...in a sane world a judge would say "get a fucking life you slimey lawyer bastards, fire up your tape duplication machines and printing press and get 100,000 copies of Donkey Kong mastered for the Commodore 64 in 48 hours or your copyright is dissolved forever" however we live in a crazy world where such common sense in our legal system does not exist and copyright on old crusty games you will never find again is enforced in this new era of information dispersal.